The immune system in higher vertebrates represents the first line of defence against various antigens that can enter the vertebrate body, including micro-organisms such as bacteria, fungi and viruses that are the causative agents of a variety of diseases. Moreover, the immune system is also involved in a variety of other diseases or disorders, including autoimmune or immunopathologic diseases, immunodeficiency syndromes, atherosclerosis and various neoplastic diseases. Although methods are available for treating these diseases, many current therapies provide less than adequate results. Among new emergent therapeutic strategies, those based on cell therapy appear to constitute a potentially useful tool for treating a great number of diseases. Thus, a great effort is being currently made by researchers in order to achieve said aim.
Autoimmune Diseases
Autoimmune diseases are caused when the body's immune system, which is meant to defend the body against bacteria, viruses, and any other foreign product, malfunctions and produces a pathological response against healthy tissue, cells and organs. Antibodies, T cells and macrophages provide beneficial protection, but can also produce harmful or deadly immunological responses.
Autoimmune diseases can be organ specific or systemic and are provoked by different pathogenic mechanisms. Organ specific autoimmunization is characterized by aberrant expression of major-histocompatibility complex (MHC) antigens, antigenic mimicry and allelic variations in MHC genes. Systemic autoimmune diseases involve polyclonal B cell activation and abnormalities of immunoregulatory T cells, T cell receptors and MHC genes. Examples of organ specific autoimmune diseases are diabetes, hyperthyroidism, autoimmune adrenal insufficiency, pure red cell anemia, multiple sclerosis and rheumatic carditis. Representative systemic autoimmune diseases are systemic lupus erythematosus, chronic inflammation, Sjogren's syndrome, polymyositis, dermatomyositis and scleroderma.
Current treatment of autoimmune diseases involves administering immunosuppressive agents such as cortisone, aspirin derivatives, hydroxychloroquine, methotrexate, azathioprine and cyclophosphamide or combinations thereof. The dilemma faced when administering immunosuppressive agents, however, is the more effectively the autoimmune disease is treated, the more defenseless the patient is left to attack from infections, and also the more susceptible for developing tumours. Thus, there is a great need for new therapies for the treatment of autoimmune diseases.
Inflammatory Disorders
Inflammation is a process by which the body's white blood cells and secreted factors protect our bodies from infection by foreign substances, such as bacteria and viruses. Secreted factors known as cytokines and prostaglandins control this process, and are released in an ordered and self-limiting cascade into the blood or affected tissues.
Inflammatory Bowel Disease (IBD)
IBD is a family of chronic, idiopathic, relapsing, and tissue-destructive diseases characterized by dysfunction of mucosal T cells, altered cytokine production and cellular inflammation that ultimately leads to damage of the distal small intestine and the colonic mucosa. IBD is clinically subdivided into two phenotypes: Crohn's disease (CD) and ulcerative colitis. CD is a nowadays incurable autoimmune disease with a prevalence of 0.05% that leads to chronic inflammation resulting in a range of gastrointestinal and extraintestinal symptoms, including abdominal pain, rectal bleeding, diarrhea, weight loss, skin and eye disorders, and delayed growth and sexual maturation in children. These symptoms can greatly impact the patients' well being, quality of life, and capacity of function. Because CD is chronic and typically has an onset before 30 years of age, patients generally require lifelong treatment. Although its etiology remains unknown, there is circumstantial evidence to link CD to a failure of the mucosal immune system to attenuate the immune response to endogenous antigens.
Therapeutic agents currently used for CD, including aminosalicylates, corticosteroids, azathioprine, 6-mercaptopurine, antibiotics, and methotrexate, are not entirely effective, nonspecific, and with multiple adverse side effects. In most cases, surgical resection is the ultimate alternative. Therefore, the present therapeutic strategy is to find drugs or agents that specifically modulate both components of the disease, i.e., the inflammatory and T-cells driven responses.
Recently, the drug infliximab has been approved for the treatment of moderate to severe Crohn's disease that does not respond to standard therapies and for the treatment of open, draining fistulas. Infliximab, the first treatment approved specifically for Crohn's disease, is an anti-tumour necrosis factor (TNF) antibody. TNF is a protein produced by the immune system that may cause the inflammation associated with Crohn's disease. Anti-TNF removes TNF from the bloodstream before it reaches the intestines, thereby preventing inflammation. However, since it has a systemic effect, and TNF is a very pleiotropic factor, severe side effects are relatively common, and its long-term safety is still to be determined. Also, the efficacy is also limited because many of the inflammatory processes that occur in the patients are not dependant on TNF signalling.
Rheumatoid Arthritis (RA)
Rheumatoid arthritis and juvenile rheumatoid arthritis are types of inflammatory arthritis. Arthritis is a general term that describes inflammation in joints. Some, but not all, types of arthritis are the result of misdirected inflammation. Rheumatoid arthritis affects about 1% of the world's population and is essentially disabling. Rheumatoid arthritis is an autoimmune disorder where the body's immune system improperly identifies the synovial membranes that secrete the lubricating fluid in the joints as foreign. Inflammation results, and the cartilage and tissues in and around the joints are damaged or destroyed. The body replaces damaged tissue with scar tissue, causing the normal spaces within the joints to become narrow and the bones to fuse together.
In rheumatoid arthritis, there is an autoimmune cycle of persistent antigen presentation, T-cell stimulation, cytokine secretion, synovial cell activation, and joint destruction.
Currently available therapy for arthritis focuses on reducing inflammation of the joints with anti-inflammatory or immunosuppressive medications. The first line of treatment of any arthritis is usually anti-inflammatories, such as aspirin, ibuprofen and Cox-2 inhibitors such as celecoxib and rofecoxib. Anti-TNF humanized monoclonal antibodies, such as Infliximab are also used; however, it has many secondary effects or side effects and its efficacy is quite low. “Second line drugs” include gold, methotrexate and steroids. Although these are well-established treatments for arthritis, very few patients remit on these lines of treatment alone, and difficult treatment issues still remain for patients with rheumatoid arthritis.
In general, the current treatments for chronic inflammatory disorders have a very limited efficiency, and many of them have a high incidence of side effects or cannot completely prevent disease progression. So far, no treatment is ideal, and there is no cure for these type of pathologies. Thus, there is a great need for new therapies for the treatment of inflammatory disorders.
Inhibition of T-Cell Responses
All immune responses are controlled by T cells. Self-reactive cells with the potential to elicit autoimmune responses comprise a part of the normal T cell repertoire, but in the healthy state, their activation is prevented by suppressor cells. Although T suppressor cells were originally described in the 1970s, significant progress in characterizing T-cell subsets has been made only recently, when they have been renamed as regulatory T cells.
There are different CD4+, CD8+, natural killer cell, and γδ T cell subsets with regulatory (suppressor) activity. Two major types of T-reg cells have been characterized in the CD4+ population, i.e., the naturally-occurring, thymus-generated T-reg cells, and the peripherally-induced, IL-10 or TGF-β secreting T-reg cells (Tr1 cells). The CD4+CD25+, Foxp3-expressing, naturally-occurring T-reg cells generated in thymus, migrate and are maintained in the periphery. The signals for their thymic generation and maintenance in the periphery are not entirely defined, although both CD28 stimulation and IL-2 appear to be required. The number of CD4+CD25+ T-reg cells in the periphery does not decrease with age, although these cells are allergic and prone to apoptosis, and their site of origin, the thymus, undergoes age-related involution. This suggests that the pool of CD4+CD25+ T-reg cells is maintained peripherally. Several experimental models support the idea of peripheral generation of CD4+CD25+ T-reg cells from CD4+CD25− T cells. The endogenous factors and mechanisms controlling the peripheral expansion of CD4+CD25+ T-reg cells are mostly unknown.
There is evidence that the cytokine transforming growth factor-beta (TGF-β) plays an important role in the expansion of thymus-derived, professional CD4+ CD25+ precursors that circulate in the blood. TGF-β is also involved in the generation of peripherally induced CD4+ and CD8+ regulatory subsets.
However, recent experimental data suggest that a mechanism of immunotolerance could be dependent on tryptophan metabolism, and in particular on the activity of the enzyme indoleamine 2,3-dioxygenase (IDO), which is an intracellular heme-containing enzyme that catalyzes the initial rate-limiting step in tryptophan degradation along the kynurenine pathway.
There is considerable evidence that supports the hypothesis that cells expressing IDO can suppress T cell responses and promote tolerance (Mellor and Munn, Nat Rev Immunol. 2004 October; 4(10):762-74). IDO is expressed in some subsets of dendritic cells (DCs), which are key regulators of immune response (tolerogenic DCs). These DCs are able of suppressing in vivo T-cell responses by locally depleting tryptophan (US Patent No. 2002/0155104). Aside from monocyte-derived DCs and macrophages, several tumour lines, intestinal cells, and trophoblasts express IDO. The expression of IDO in trophoblasts appears to be constitutive and has been strongly correlated to tolerance of allogeneic tissue from the foetus. IDO is believed to induce apoptosis in T cells, and cause spontaneous tolerance to liver allografts.
The molecular mechanisms behind the immunosuppressive activity of IDO are not known. However, it has been demonstrated that DCs expressing IDO are able to induce the generation of regulatory T cells. IDO is induced in human cells by several inflammatory mediators, including interferons and lipopolysaccharide (LPS), as well as by viral infection. Several studies have shown that allogeneic tumour cells being rejected by the host immune system in vivo up-regulate IDO and this effect is mediated by IFN-γ.
Recent experiments have indicated an in vitro immunosuppressive capacity of bone marrow derived mesenchymal stem cells (MSCs) and adipose-derived stem cells (ASCs), as well as an in vivo immunosuppressive capacity of MSCs. This in vivo activity has been studied in bone marrow transplants, in which the infusion of expanded MSCs appears to reduce acute and chronic graft versus host disease (GVHD). The in vitro effect is characterized by a suppression of lymphocyte proliferation in experiments where the lymphocytes were activated either via a mixed lymphocyte reaction (MLR) or stimulation with phytohemagglutinin (PHA). However, the molecular mechanisms responsible for the immunosuppressive effects of said cells have not been unequivocally identified.